1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine seismic surveys.
2. Description of the Related Art
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for hydrocarbon deposits located in subterranean formations. In seismic surveying, seismic energy sources are used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors. Such seismic reflectors typically are interfaces between subterranean formations having different elastic properties, specifically wave velocity and rock density, which lead to differences in acoustic impedance at the interfaces. The reflections are detected by seismic receivers at or near the surface of the earth, in an overlying body of water, or at known depths in boreholes. The resulting seismic data may be processed to yield information relating to the geologic structure and properties of the subterranean formations and their potential hydrocarbon content.
The goal of seismic data processing is to extract from the data as much information as possible regarding the subterranean formations. In order for the processed seismic data to accurately represent geologic subsurface properties, the reflection amplitudes need to be represented accurately. Non-geologic effects can cause the measured seismic amplitudes to deviate from the amplitude caused by the reflection from the geologic target. Amplitude distortions resulting from irregular distribution of source and receiver positions during data acquisition is a particularly troublesome non-geologic effect. If uncorrected, these non-geologic effects can dominate the seismic image and obscure the geologic picture.
A seismic wave source generates a wave that reflects from or illuminates a portion of reflectors at different depths. The reflected seismic wave is detected by sensors and the detected signals are recorded. In a three-dimensional (3D) survey, seismic signals are generated at a large number of source locations and the survey generally illuminates large regions of the reflectors. Conventional prestack 3D migration algorithms can produce good images of the sub-surface horizons only if the surface distribution of sources and receivers is relatively uniform. In practice, there are always irregularities in the distribution of sources and receivers. Obtaining perfectly regular acquisition geometry is typically too expensive. Consequently, prestack 3D migrated images are often contaminated with non-geologic artifacts. These artifacts can interfere with the interpretation of the seismic image and attribute maps. A goal in seismic acquisition is balancing the regularity of source and receiver distribution with reasonable acquisition cost.
In marine seismic streamer surveys, the streamers do not form straight lines. Typically marine currents cause the streamers to curve, a phenomenon called feathering and the curvature is typically measured in degrees. Changes in the currents often cause changes in the feathering. In such circumstances, if the planned sail line separation of the seismic vessel is maintained, then feathering will lead to coverage holes at some offsets or offset ranges, at some depths. The term “coverage hole” as used herein refers to a surface area where, for a given offset or offset range, there are inadequate data recorded. Data are defined to be located at the surface midpoint positions between source and receiver pairs. The coverage holes can be of several kilometers extension in the sail line (inline) direction, but are of the order of ten to a few hundred meters in the (cross line) direction orthogonal to the sail line.
In marine seismic streamer surveys, portions of the surface are often not adequately covered with receiver recordings due to cable feathering. Thus, in order to cover these areas that were missed on the first pass, additional passes of the seismic vessel through the prospect survey area have been required. Additional numbers of sail-lines can also arise from steering the vessel to achieve acceptable coverage. That means that the distance between passes in on average less than in the original acquisition plan. These additional passes significantly increase the time and associated cost to complete a survey. These additional passes of the survey vessel are referred to as “infill shooting”. A large portion of marine seismic data collection can be devoted to the infill shooting portion of a survey. The infill portion may take up to several weeks or even months to complete. Thus, it is not uncommon to spend 15-20% of total acquisition costs on infill acquisition. Any reduction in these large infill costs would provide an economic advantage.
Maximum data hole sizes that will provide acceptable subsurface coverage are typically determined prior to acquisition, and are typically independent of local factors such as geology and survey objectives. Criteria for a seismic survey, such as acceptable subsurface coverage, are commonly called “infill specifications”. In the past, evaluating whether a survey acquisition plan will provide acceptable subsurface coverage has been done during, or after the acquisition takes place. Waiting until after acquisition, however, means either incurring the cost of retaining equipment and personnel at the survey area until the evaluation is made or risking having to return equipment and personnel to the survey area for additional infill acquisition at considerable cost. Making infill acquisition decisions during acquisition means being able to commence additional infill acquisition without waiting. For example, Brink, M., Jones, N., Doherty, J., Vinje, V., and Laurain, R., “Infill decisions using simulated migration amplitudes”, SEG Int'l. Exp. and 74th Ann. Mtg., Denver, Colorado., Oct. 10-15, 2004, pp. 57-60 describe a method for making infill decisions during acquisition. The seismic data are modeled in a velocity depth model using navigation data and migration amplitudes along key horizons. The navigation data and velocities can be acquired during the acquisition and then the simulated migration amplitudes can be generated during the acquisition. The need for further infill shooting can then be determined.
However, it would be more efficient to determine the maximum acceptable coverage hole sizes before acquisition begins. Then, any deficiencies discovered could be corrected during acquisition, reducing considerably the need for additional infill acquisition afterwards. However, Brink et al. 2004 does not disclose how to make infill specifications relating to data hole coverage before acquisition begins.
A three-part series—Muerdter, D., and Ratckiff, D., “Understanding subsalt illumination through ray-trace modeling, Part 1: Simple 2D salt models”, The Leading Edge, Vol. 20, Issue 6, June, 2001, pp. 578-594, (Muerdter et al. 2001a); Muerdter, D., Kelly, M., and Ratckiff, D., “Understanding subsalt illumination through ray-trace modeling, Part 2: Dipping salt bodies, salt peaks, and nonreciprocity of subsalt amplitude response”, The Leading Edge, Vol. 20, Issue 7, July, 2001, pp. 688-697, (Muerdter et al. 2001b); and Muerdter, D., and Ratckiff, D., “Understanding subsalt illumination through ray-trace modeling, Part 3: Salt ridges and furrows, and the impact of acquisition orientation”, The Leading Edge, Vol. 20, Issue 8, August, 2001, pp. 803-816, (Muerdter et al. 2001c)—describe the application of ray-trace modeling to clarify imaging problems under various salt structures such as salt sheets and detached irregularly-shaped salt bodies. The modeling comprises building 3D salt shape and velocity models, applying ray-tracing to 2D and 3D prestack depth migration surveys, and then sorting the data into common reflection point (CRP) gathers for comparison to the migrated seismic data. Muerdter et al. 2001c claim that modeling can be used to predict the expected illumination and determine the best acquisition parameters before acquisition, but the only parameter studied is the effect of acquisition orientation (shooting direction) relative to structural orientation (ridges and troughs) of the salt structure (although offset length is also mentioned but not discussed in Muerdter et al. 2001a). However, Muerdter et al. 2001a do not disclose how to make infill specifications relating to data hole coverage as surface coverage before acquisition begins.
Thus, a need exists for a method for a priori determination of the sufficiency of acquisition coverage for a given marine seismic streamer survey, that is, for determining the size of coverage holes that are acceptable before the acquisition takes place.